PYRO-------- Black-body and Non-black-bo(* calibrated to read correctly when s approximate black-body conditions j in the open, certain corrections convert them into true temperatures. In case the temperature of one section of an unequally heated furnace is required or if the furnace contains much smoke or dust it is frequently desirable to sight the radiation pyrometer into the bottom of a fireclay or porcelain tube as illustrated by Fig. 25. The tube should be uniformly heated at the end for a length at least three times its diameter. The tube must have a diameter such that the cone of rays entering the telescope is not intercepted by the front of the tube. Figure 26 illustrates this condition. Suppose the distance from the bottom of the tube to the mirror of the Fery pyrometer is 100 cm. Referring to Table 17, the diameter of the source re- quired is 4.8 cm. Lay off the distance b = _ _. _ Ulaw B unes from the bottom of b to the bottom of the mirror c, and from the top of b to the top of the mirror. The tube must have such a diameter at its front end a that it does not cut in on the cone of rays represented by these two straight lines. Phis can be determined only by actually making the above drawing to scale or by computing. It can not be decided by looking through the FIG. 25.—Use of ™ „„ observing temperature. ; 4.8 cm. and draw straight lines FIG. 20.—Porcelain tube for observing temperatures. telescope of the pyrometer since the front end of the tube would not necessarily show even if it did cut in on the cone of rays. In the case of the fixed-focus instruments the diameter of the tube must be such that the cone of rays cuts the tube in the region which is uniformly heated. The following table shows the true temperatures corresponding to the apparent or measured temperatures observed with a radiation pyrometer when sighted upon yanous materials in the open. This table must not be confused with Table 12 for the optocal pyrometer. It will be noticed that the corrections are entirely different lor the two types of pyrometer.